N-dealkylation of tertiary amines is a key chemical transformation in many processes for the preparation of clinically and commercially important compounds. Methods for N-dealkylation of tertiary amines are known in the art and include reaction of the tertiary amine with cyanogen bromide (see, e.g., U.S. Pat. Nos. 3,254,088 and 3,433,791; and Cooley et al., “Amine Dealkylations with Acyl Chlorides” (1989) Synthesis 1-7), dialkyl azodicarboxylates including diethylazodicarboxylate and di-iso-propylazodicarboxylate, (see, e.g., GB 1,124,441), and haloformate reagents, including vinyl, methyl, ethyl, allyl, propyl, heptyl, phenyl, benzyl, α-chloro-ethyl, and 2,2,2-tri-chloro-ethyl chloroformates (see, e.g., U.S. Pat. Nos. 3,905,981 and 4,472,253; Olofson et al. (1984) J. Org. Chem. 49(11):2081-2083; and Rice et al. (1975) J. Org. Chem. 40(12):1850-1851).
Additional methods for N-dealkylation, particularly N-demethylation of tertiary amines, involve photochemical cleavage, as well as the formation and hydrolysis of dithiocarbamate, methyoxymethylether, and amine N-oxide intermediates to provide the corresponding secondary amine derivatives (see, e.g., Santamaria et al. (1989) Tetrahedron Lett. 30:2927; Santamaria et al. (1990) Tetrahedron Lett. 31:4735; Acosta et al. (1994) J. Chem. Soc., Chem. Commun. 17(7):1985-1986; Murahashi et al. (1988) J. Am. Chem. Soc. 110:8256; Murahashi (1995) Angew. Chem., Int. Ed., Engl. 34:2443; Polniaszek et al. (1992) J. Org. Chem. 57:4103; Murahashi et al. (1992) Tetrahedron Lett. 33:6991; Murahashi et al. (2003) J. Am. Chem. Soc. 125:15312; McCamley et al. (2003) J. Org. Chem. Soc. 68:9847; Gesson et al., “Preparation of N-Demethyl and N-Alkyl Analogs of L-Rhodosamine” (November 1990) Synlett. 669-670; Rosenau et al. (2004) Org. Lett. 6:541; Menchaca et al. (2003) J. Org. Chem. 68:8859; Periasamy et al. (2000) J. Org. Chem. 65:3548; Saaby et al. (2000) Angew. Chem., Int. Ed., Engl. 39(22):4114-4116; Denis et al. (2002) Tetrahedron Lett. 43:4171; and Zhang et al. (2005) Org. Lett. 7:3239).
In particular, methods for the preparation of noroxymorphone and noroxycodone involve removal of the naturally occurring opioid N-methyl group to provide the corresponding secondary amine. Methods for the preparation of other semi-synthetic opiate derivatives, e.g., naloxone, naltrexone, nalorphine, nalmefene, and nalbuphine, involve removal of that naturally occurring opioid N-methyl group and its replacement with another alkyl or an alkenyl moiety. The ultimate starting materials for preparation of these semi-synthetic compounds include the natural products morphine, codeine, thebaine, and oripavine. Among these, thebaine and oripavine are particularly useful because they are readily oxidized to introduce the 14-hydroxyl group carried by each of the above semi-synthetic opiates. In a similar manner, the semi-synthetic processes for the synthesis of buprenorphine, levallorphan, pentazocine, cyclazocine, and ketazocine also involve replacement of an N-methyl group of a tertiary amine with an alkyl or an alkenyl moiety. Synthesis of cabergoline from either lysergol or elymoclavine also involves replacement of a tertiary N-methyl group with an allyl moiety.
As described in the above references, the tertiary amine is converted to an intermediate that is subsequently cleaved to provide the corresponding dealkylated (demethylated) secondary amine. The secondary amine can then be realkylated, e.g., by condensation with an alkyl or alkenyl halide selected from among propyl iodide, cyclopropyl methyl bromide, cyclobutyl methyl bromide, and allyl bromide (see, e.g., U.S. Pat. Nos. 3,905,981; 4,141,897; 3,254,088; 3,332,950; and 3,433,791).
However, these reactions can involve the use of materials and reagents that are relatively expensive, toxic and environmentally burdensome. Furthermore, the synthetic schemes disclosed generally involve three steps for conversion of a tertiary amine to the corresponding N-allyl derivative, as noted above. Such processes also may require purification of intermediates, extended process times, and harsh reaction conditions, and may provide overall yields that are not commercially viable.
Accordingly, there remains a need for more efficient methods for the preparation of N-allyl derivatives of tertiary amines, as well as for improved processes incorporating those methods that would be robust, cost effective, amenable to commercial scale-up, and/or that would impose lower burdens on the environment. In particular, there remains a need for more efficient methods for preparing semi-synthetic opiate derivatives including, e.g., naloxone, naltrexone, nalmefene, nalbuphine, noroxymorphone, noroxycodone, and buprenorphine, as well as levallorphan, pentazocine, cyclazocine, ketazocine, and cabergoline.